1. Field of the Invention
The present invention relates to an organopolysiloxane composition that can be vulcanized at room temperature into an elastomer that is crosslinked by polycondensation and that does not contain alkyltin-based catalysts which exhibit toxicity problems.
2. Description of Related Art
The invention also relates to novel polycondensation catalysts having a guanidine structure, in silicone chemistry, and to the uses thereof as catalysts for the organopolysiloxane polycondensation reaction.
Formulations of elastomers that crosslink via polycondensation generally involve a silicone oil, generally a polydimethylsiloxane, with hydroxyl end groups, optionally prefunctionalized with a silane so as to have alkoxy ends, a crosslinking agent, a polycondensation catalyst, conventionally a tin salt or an alkyl titanate, a reinforcing filler and other optional additives such as bulking fillers, adhesion promoters, colorants, biocidal agents, etc.
These room-temperature vulcanizing organopolysiloxane compositions are well known and are classified into two different groups: single-component (RTV-1) compositions and two-component (RTV-2) compositions.
The term “RTV” is the acronym for “Room Temperature Vulcanizing”.
During crosslinking, water (either provided by atmospheric moisture in the case of RTV-1 compositions, or introduced into a part of the composition in the case of RTV-2 compositions) enables the polycondensation reaction, which results in the formation of the elastomeric network.
Generally, single-component (RTV-1) compositions crosslink when they are exposed to moisture from the air, i.e. they cannot crosslink in an enclosed medium. For example, the single-component silicone compositions used as sealants or adhesives cold-crosslink according to a mechanism of hydrolysis of reactive functions of the acetoxysilane, ketiminoxysilane, alkoxysilane, etc., type, followed by condensation reactions between silanol groups formed and other residual reactive functions. The hydrolysis is generally carried out by virtue of water vapor which diffuses into the material from the surface exposed to the atmosphere. Generally, the kinetics of the polycondensation reactions is extremely slow; these reactions are therefore catalyzed by a suitable catalyst. As catalysts which are employed, use is most often made of catalysts based on tin, titanium or an amine or compositions of these catalysts. Catalysts based on tin (cf. in particular FR-A-2 557 582) and on titanium (cf. in particular FR-A-2 786 497) are catalysts that are very effective. Single-component silicone elastomers with —Si(OR) ends are sometimes referred to as alkoxy elastomers.
As regards two-component compositions, they are sold and stored in the form of two-components, a first component containing the base polymer materials and the second component containing the catalyst. The two components are mixed at the time of use and the mixture crosslinks in the form of a relatively hard elastomer. These two-component compositions are well known and are described, in particular, in the book by Walter Noll “Chemistry and Technology of Silicones” 1968, 2nd edition, on pages 395 to 398. These compositions essentially comprise four different ingredients:                an α,ω-dihydroxydiorganopolysiloxane reactive polymer,        a crosslinking agent, generally a silicate or a polysilicate,        a tin catalyst, and        water.        
Usually, the condensation catalyst is based on an organic tin compound. Indeed, many tin-based catalysts have already been proposed as crosslinking catalysts for these RTV-1 or RTV-2 compositions. Conventional polycondensation catalysts comprise dialkyltin compounds, in particular dialkyltin dicarboxylates such as dibutyltin dilaurate and dibutyltin diacetate, alkyl titanate compounds such as tetrabutyltitanate or tetra-isopropyltitanate, and titanium chelates (EP-A-0 885 933, U.S. Pat. No. 5,519,104, U.S. Pat. No. 4,515,932, U.S. Pat. No. 4,563,498, U.S. Pat. No. 4,528,353).
However, the alkyltin-based catalysts, although very effective, usually colorless, liquid and soluble in silicone oils, have the drawback of being toxic (CMR2 toxic for reproduction).
Thus, international application WO 2004/020525 describes single-component (RTV-1) silicone compositions used as sealants or adhesives and which cold-crosslink when they are exposed to the moisture in the air and which comprise, in addition to the usual components:                a specific and essential crosslinking agent (D) which is a silane comprising 1-methylvinyloxy functions, known for its strong reactivity compared with that of the conventional crosslinking agents, and        catalysts which are organic derivatives comprising imine functions, of formula (I) or (II) below:        
with R being a specific radical chosen from the following groups: methyl, isopropyl, phenyl and ortho-tolyl. Examples of these organic derivatives of imine type are 1,3-diphenylguanidine, 1,3-di-o-tolyl-guanidine, 1,3-dimethylguanidine and 1,1,3,3-tetra-methylguanidine, which is the preferred derivative. These derivatives have the particularity of possessing an unsubstituted imine function, i.e. a function of the C═NH type.
It should be noted that a conventional crosslinking agent of trialkoxysilane type, component (E), is still used in combination with the crosslinking agent (D) which is a silane known for its strong reactivity due to the presence of functions of 1-methylvinyloxy type.
However, the problem associated with the use of the organic catalysts comprising imine functions described in international application WO 2004/020525 is that they must be used in the presence of specific crosslinking agents that are very reactive and expensive (silanes comprising 1-methylvinyloxy functions), that is to say that conventional crosslinking agents having simple structures, which are very widely used in RTV-1 or RTV-2 formulations, for instance alkyltrialkoxysilanes, alkyl silicates or poly(alkyl silicate)s, cannot be combined with them without the obligatory presence of a very reactive crosslinking agent such as the silane with 1-methyl-vinyloxy functions. This is because, without the presence of this very reactive silane, the crosslinking of the composition into an elastomer is then insufficient and does not make it possible to obtain good mechanical properties. Thus, the 1,1,3,3-tetramethylguanidine derivative, which is presented in the preferred embodiment of this patent application, when it is used with a conventional crosslinking agent, for instance a poly(alkyl silicate), and without the presence of a reactive silane comprising methylvinyloxy functions, in a single-component RTV (RTV-1), the crosslinking of the system is then insufficient and cannot generate a silicone elastomer.
These problems with reactivity of the crosslinking agent, for example, in single-component (RTV-1) silicone compositions are well known by those skilled in the art. Indeed, the alkoxysilane crosslinking agents most widely used are those which have methoxy groups for their intrinsic reactivities. However, one of the problems associated with the use of alkoxysilanes of this type is that methanol is given off and is a source of problems in terms of health and safety.
For a sustainable development, it therefore appears to be necessary to develop other, nontoxic, catalysts for the organopolysiloxane polycondensation reaction which can be used in the crosslinking of both single-component elastomer compositions and two-component elastomer compositions. In addition, these catalysts should be usable irrespective of the type of crosslinking agent used.
Another important aspect for an organopolysiloxane polycondensation reaction catalyst is the working time (pot-life), i.e. the time during which the composition can be used after mixing, without it curing. This time should be sufficiently long to allow its use, but sufficiently short to obtain a handleable molded object at the latest a few minutes or a few hours after it has been produced. The catalyst should therefore make it possible to obtain a good compromise between the time during which the catalyzed mixture can be used and the time after which the molded object is handleable (these times depend on the intended application, for instance the molding or the production of seals). In addition, the catalyst should confer, on the catalyzed mixture, a spreading time which does not vary according to the storage time.